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Being that it is "ultrasonic," I expect it's efficiency depends upon the rigidity of the material being cut. It would operate similar to a Fein multitool, or the various knock-offs of that which operate on the principle of an orthopedic cast saw. The minutely vibrating saw blade will cut the rigid plaster cast, but not the skin beneath. Up against hard styrene plastic, it pretty much does quite well. I wonder how a hobby knife blade, designed for cutting, now sawing, would do in it when cutting wood, and particularly soft wood, which is cut by shearing in that application. If a fine-toothed saw blade were available for it, I expect it would saw like a miniature multitool saw blade.

 

It probably costs a fair amount. The question would be does it do the job better?  Is it more accurate or does it work more quickly? Does it produce better or faster work? Or is it just another gizmo? It's clearly a "mousetrap," but is it a "better mousetrap?"

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This principle of this knife is the "slicing effect".  The most common example is cutting a tomato - if you push the knife straight in, it won't cut, but if you add a movement to the knife 90 degrees to the direction you want to go (add a slicing action), then the cutting forces are greatly reduced.  The amount that the forces are reduced depends on the speed of the slicing movement relative to the pushing speed.  The faster the knife moves, the lower the forces.   The effect has nothing to do with the material properties, it's just geometry in that the slicing movement reduces the sharpness angle of the knife, as seen by the material being cut.

 

There is a limit to how much the slicing speed will reduce the forces because there is friction and  for an oscillating knife, which has to stop at both ends of the stroke so the full forces are needed to push the knife forward.  The quoted 40,000 rpm is probably way more than needed.  

 

If you want to try a home-made version, attach a knife to the working end of a beard trimmer, electric hair clipper or an electric engraver.  I suspect one of the main design problems with the Wonder-cutter is balancing it to avoid the fingers going numb and to provide better control.   I've made a bigger version using a sharpened scraper on an oscillating multi-tool (Fein, for example).  I was able to slice off 1/8 inch thick pieces with a roughly 80% reduction in cutting force.

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